1. Field of the Invention
This invention relates generally to power supplies, and more particularly, to the type of power supply that converts an input DC voltage to an output DC voltage having a different magnitude.
2. Description of the Related Art
DC to DC converters have the property that a constant input voltage level can be converted to a constant output voltage level having a different magnitude, i.e. a DC input voltage can be converted to a higher DC output voltage or to a lower DC output voltage. In addition, the output voltage can be electrically isolated from the input voltage.
The present invention generally has application in converting higher DC voltage, applied to the DC to DC converter from the power distribution source, to a lower DC voltage. In particular, in a data processing system, the distribution of power at higher voltage levels reduces the I.sup.2 R losses and reduces the effect of contact resistances of the distributing conducting path on the voltage level reaching the local power source. The lower DC voltage level from the DC to DC converter is compatible with the requirements of the circuit elements used in the data processing system.
One implementation technique for prior DC to DC converters involves the use of sinusoidal primary and sinusoidal secondary voltages and is referred to as a resonant DC to DC converter. Resonant converter circuits require an inductor, a capacitor and very fast diode clamps in the primary circuit (elements that are not required in the quasi-resonant DC to DC converter circuits described below). The resonant converter circuit has the advantage that the sinusoidal voltages reduce the switching losses in the field effect transistors and that the operation is consistent with high frequency operation. The high frequency operation environment permits the use of physically small transformer components and secondary filter circuit components. However, the high frequency operation also induces high dV/dt transients into the switching circuit causing the field effect transistors to experience gate drive problems. Furthermore, the primary switch device must handle larger than necessary switch currents. The circuit may require a Schottky diode in series with the field effect transistor as well as a high voltage 30 ns, 3 ampere diode in parallel with the Schottky diode and series field effect transistor. The use of resonant circuits can also result in a lack of stability when a high Q circuit is used (i.e. to minimize energy losses.) Finally the use of resonant circuits, with the inherent transfer of energy between components of the tank circuit, is fundamentally inconsistent with the unidirectional delivery of power to the components energized by the DC to DC converter.
More recently, Vinciarelli, in U.S. Pat. No. 4,415,959 and in U.S. Pat. No. 4,441,146 has described a quasi-resonant circuit. In the quasi-resonant circuit configuration, the leakage inductance of the transformer coupling the input and output portions of the converter forms, in combination with an external high Q capacitor, a resonant circuit. A capacitor between the leakage inductance of the transformer and the external capacitor, prevents the actual resonance of the circuit. The resonant frequency of the circuit is typically not the frequency at which the input terminals of the transformer are being activated. In addition, the circuits described in U.S. Patents by Vinciarelli have a single ended configuration, i.e. a single rectifying element in the secondary circuit of the transformer, and consequently operates at reduced power level. In U.S. Pat. No. 4,441,146, a circuit is described that resets the magnetic flux in the transformer arising from the single-ended implementation.
A need has therefore been felt for a DC to DC converter circuit that provides the advantages of the quasi-resonant DC to DC converter without the limitations of the prior devices.